1. Field of the Invention
The present invention relates to the preparation of small silicon particles that are suitable for use as seed particles in a fluidized bed reactor for producing high-purity polycrystalline silicon (polysilicon"). More particularly, the present invention relates to a method for pulverizing silicon particles wherein silicon feed particles are introduced into a pulverizing chamber, collide with each other and pulverize by fluid jet energy to prepare small silicon seed particles without contamination.
2. Description of the Background
As a typical method of producing polysilicon, the Siemens process has widely been used wherein silicon is deposited on a heated silicon rod by the electrical resistance method in a bell jar. However, the productivity of silicon is low since the reaction area of the Siemens reactor is restricted to the surface of the silicon rod. In addition, the cooling necessary for avoiding silicon deposition on the surface of the bell jar lowers thermal efficiency. Therefore, the polysilicon produced by the Siemens process is expensive.
To overcome the foregoing disadvantages, many efforts have been made on a fluidized bed process which has a larger reaction area and a higher yield than the Siemens process and thereby can produce polysilicon with lower cost. Ethyl Corporation now produces semiconductor-grade polysilicon granules commercially by a fluidized bed process using monosilane as a source gas. According to the fluidized bed process, the seed silicon particles charged within a reactor are fluidized with hydrogen and the silicon source gas, e.g., monosilane, trichlorosilane, or the like. Silicon is deposited on the surface of said silicon particles to produce polysilicon by the thermal decomposition or hydrogen reduction of the silicon source gas.
To operate a fluidized bed reactor continuously, the quantity and size of silicon particles within the reactor must be maintained in a predetermined range. For this purpose, the grown silicon particles should be recovered from the bottom of the reactor as polysilicon products while small silicon seed particles should be charged into the reactor continuously.
As a conventional method for preparing the silicon seed particles for the fluidized bed process, a method is available which comprises crushing silicon lumps or particles by mechanical apparatus, sieving, repeated acid washing and rinsing, and drying. In this method the wet process of washing and rinsing is very important due to considerable contamination from abrasion of the apparatus. However, it is most difficult to obtain seed particles of the required purity by the wet process.
Improved methods for preparing silicon seed particles with less contamination have also been developed. Two of the methods are disclosed in laid open Japanese Patent Publication No. 58-145,611 and U.S. Pat. No. 4,691,860. According to the former publication, silicon nuggets or particles are pulverized between two rollers made of high purity silicon bar, and then the pulverized particles are sieved to have a predetermined size range, thereby preparing small silicon seed particles of high purity. However, this method has several disadvantages. For example, the silicon bars are greatly abraded, many particles are produced outside of the desired size range, and the sieving system is comparatively complicated. The method disclosed in the latter publication comprises transporting the silicon particles in a gas flow to accelerate and split them by collision with a silicon plate. This method also can produce high-purity silicon seed particles, but it is necessary to replace the silicon colliding plate with a new one periodically due to serious abrasion. In addition, the yield of the seed particles of the desired size range is low since silicon particles are pulverized by single collision with the plate.
Other than the aforementioned methods, a fluidized-bed jet pulverizing method has conventionally been used to pulverize solid particles finely wherein they collide with each other by fluid jet energy within a fluidized bed. This method pulverizes particles by filling a pulverizing chamber with solid particles to a certain level for the purpose of increasing collisions between the particles. In experiments by the present inventors to pulverize silicon particles of about 300-3,000 microns by this method, it was found that the pulverizing efficiency was low and that an excessive quantity of fine powders that are unsuitable for use as seed particles were generated. The low efficiency is probably the result of the phenomenon that the formation of a jet stream is interrupted by the high volumetric density of silicon particles circulating around the jet nozzle. This result may also be explained by the theory that the pulverizing efficiency suddenly drops when the volumetric density of solid particles in the pulverizing zone becomes greater than about 0.1. See the article by Tanaka at pages 213-215 of Ind. Eng. Chem. Process Des. Develop., Vol. 12, No. 2 (1973). The generation of excessive fine powders is a natural outcome since this method has been used for the purpose of preparing fines below several tens of microns.
For these and other reasons, it is apparent that conventional pulverizing methods are not efficient and economical for preparing the small silicon particles with high purity which are suitable for use as seeds in a fluidized bed reactor to prepare high purity polysilicon. Therefore, it is an object of the present invention to provide an improved jet pulverizing method for preparing silicon seed particles without contamination by colliding silicon particles with each other. It is a further object of the present invention to provide an improved jet pulverizing method that can suppress the generation of fine powders that are unsuitable for use as seed particles. It is a further object of the present invention to provide an improved jet pulverizing method by which silicon feed particles may be fed into a pulverizing chamber automatically without an additional feeding apparatus. It is an additional object of the present invention to provide an improved jet pulverizing method which can recover pulverized particles without additional sieving to separate and remove particles larger than the desired size.